Electromagnetic operating mechanisms include a magnet yoke, an operating coil, and an armature which is attracted by the magnet yoke when the operating coil is sufficiently energized. Electromagnetic operating mechanisms are used in electromagnetic switching devices (also referred to as contactors) for connecting and disconnecting an electric load to and from an electric power network by closing or opening the main contacts coupled to the armature. For reasons of safety, the regulations relevant to these switching devices require the load to be disconnected from the network when the control input of the electromagnetic operating mechanism is de-energized.
Therefore, electromagnetic switching devices usually have electromagnetic operating mechanisms which hold the main contacts open using return springs when the operating coil is in the de-energized state. It is a disadvantage of such electromagnetic operating mechanisms that, in order to maintain the main contacts closed, a holding current through the solenoid coil, and thus a holding power, are required, as result of which waste heat is generated during operation, requiring the electrical system to be thermally designed accordingly.
German Publication DE 101 29 153 A1 describes an electromagnetic valve in which a higher pull-in current is reduced to a lower holding current. The solenoid coil field or the solenoid coil current, which change when the valve is switched, are detected by sensor means in the form of a magnetic field sensitive switch or a current sensor for the coil current in order to change to the holding current. German Publication DE 299 09 901 describes a microprocessor control for an electromagnetic operating mechanism, where the holding current is minimized through pulse-width control. An electromagnetic switching device known from DE 39 08 319 A1 has a permanent magnet in the magnet yoke in order to reduce the pull-in power and holding power required. German Patent DE 101 33 713 C1 describes an electromagnetic operating mechanism which also has a permanent magnet in the magnet yoke and in which the required holding force is provided by said permanent magnet alone. When disconnecting the control voltage, a mechanical locking mechanism, which has been held by an auxiliary electromagnetic operating mechanism up to that point, is released, as a result of which a spring force counteracting the permanent magnet is released by the mechanical locking mechanism in order for the armature to drop out. However, the electromagnetic operating mechanisms mentioned above still require considerable holding power and auxiliary power, respectively.
European Patent EP 0 721 650 B1 discloses a bistable magnetic actuator having permanent magnets disposed between a magnet yoke and a two-piece armature and including two individually energizable solenoid coils. A low reluctance flux path and a high reluctance flux path are formed in each of the bistable positions of the armature. Energization of the solenoid coil linked to the high reluctance flux path causes the armature to move from one stable position to the other, thus swapping over the low reluctance flux path and the high reluctance flux path. In an electromagnetically controlled valve operating mechanism according to EP 0 376 715 B1, the holding state is brought about solely by a permanent magnet in the magnet yoke. On the other hand, the pulling in and dropping out of the armature is brought about by the suitably polarized brief discharging of a storage capacitor which has been charged in the preceding dropped-out state or holding state. German Publication DE 199 58 888 A1 describes a so-called remanent actuator, whose armature assumes the OFF position, on the one hand, and the ON position, on the other hand, between two permanent magnets disposed opposite each other and in oppositely poled relationship in the magnet yoke. The armature is moved from one position to the other and vice versa by briefly charging and discharging a capacitor, respectively. German Publication DE 201 13 647 U1 describes an electromagnetic operating mechanism for an electromagnetic switching device, said electromagnetic operating mechanism also having a permanent magnet which is disposed in a double-circuit magnet yoke and which provides the holding force alone. A storage capacitor charged during the holding mode is discharged through the auxiliary circuit in order for the armature to drop out. In the electromagnetic operating mechanisms mentioned above, no measures are taken for the armature to reliably drop out when the control power fails.
German Publication DE 101 46 110 A1 discloses an electronic circuit arrangement having a microcontroller for switching an electromagnetic operating mechanism from the electromagnetic pull-in mode to the permanent-magnetic holding mode. When the control voltage is removed, the brief discharge current of a storage capacitor is used to demagnetize the magnetic circuit, and thus, to cause the armature to drop out. The document does not specify any means to prevent the electromagnetic operating mechanism from remaining in the holding mode in the event of a defect of the circuit arrangement. A microprocessor-controlled trip solenoid of a circuit breaker having permanent-magnetic holding force is known from DE 199 54 037 A1. In order to test the holding force, the trip coil is loaded with brief current pulses at regular intervals. In case of a decrease in the holding force, tripping is performed prematurely for reasons of safety.